keyboard short cut 'E' and 'O' of terminal emulator 'terminator'
Solution 1:
The manpage uses bold formatting to show why:
Ctrl+Shift+O
Split terminals Horizontally.
Ctrl+Shift+E
Split terminals Vertically.
The second letters in "horizontally" and "vertically" are o and e respectively. Since this is how it was since the keyboard shortcuts were first added to the manpage in 2008 (before the 0.10 release), I think that's pretty much it.